(smile) `SmileGenerator.writeNumber(String)` validates number length against default `StreamReadConstraints` instead of configured ones

\`SmileGenerator._writeIntegralNumber()\` / \`_writeDecimalNumber()\` (used by \`writeNumber(String)\`) guard against too-long number Strings via \`_streamReadConstraints().validateIntegerLength()\` / \`validateFPLength()\`.

But \`_streamReadConstraints()\` returns \`StreamReadConstraints.defaults()\`, so a \`maxNumberLength\` configured on the \`SmileFactory\` is ignored on the write path — both a lower limit (not enforced) and a higher limit (spuriously rejected).

Since \`IOContext\` carries the factory's \`StreamReadConstraints\` (2.15+), the generator can use \`_ioContext.streamReadConstraints()\` instead.